The present invention relates to an apparatus for recharging of polycrystalline silicon granules into a crucible in a Czochralski single crystal growing process, hereinafter referred to as the CZ process, especially of semiconductor silicon.
As is well known, most semiconductor-grade silicon single crystals are manufactured by the so-called CZ process in which a single crystal rod of silicon is grown from a melt of silicon, contained in a fused silica glass crucible, on the lower end of a seed crystal which is pulled up at a controlled rate. This CZ process is a typical batch-wise process and it is traditional that, when most of the silicon melt has been converted into the single crystal rod, the process is discontinued and a next run is started anew. Such a process is of course not quite satisfactory with respect to productivity, even without consideration of the problem that the expensive fused silica glass crucible cracks when it is cooled down.
In this regard, it has been practiced recently that, when a single crystal rod of silicon has been grown and the amount of the silicon melt remaining in the crucible has been decreased below a certain level, the crucible is recharged with polycrystalline silicon granules through a feed conduit installed above the crucible without cooling the crucible. Such recharging is in such an amount as to enable starting of another run of single crystal growing. Further, a continuous recharging method is known in which supply of silicon granules is conducted continuously, while a silicon single crystal is still being grown, at a controlled rate of, for example, 0.3 to 1.0 g/second to compensate for the decrease in the amount of the melt in the crucible. This method provides a possibility of greatly reducing the manufacturing costs of silicon single crystals.
The above mentioned continuous recharging method has several problems. For example, falling of the silicon granules onto the surface of the silicon melt in the crucible sometimes causes splashing of the melt or vibration of the melt surface. This greatly disturbs the single crystal growing process in that the silicon single crystal as grown, if it could even be obtained, would suffer from the occurrence of so many dislocations or imperfections as to be contrary to the object of cost reduction. This problem due to disturbance of the melt surface by the falling silicon granules can of course be avoided by using a double-walled crucible in which the melt surface to which the silicon granules fall is isolated from the melt surface for the single crystal growing. Such method, however, is not practicable due to the expense of such a crucible of special structure.
Semiconductor Silicon Crystal Technology by F. Shimura, pages 178-179 (1989) teaches a multiple CZ growth method for reduction of the manufacturing costs of semiconductor silicon single crystals by recharging of silicon granules in the conventional batch-wise CZ process. In this method, a single crystal rod after completion of growth is removed from above the crucible. Polycrystalline silicon in the form of a rod is introduced, without decreasing the temperature, into the silicon melt remaining in the crucible and is melted therein before start of another run of the CZ process. Thus, a fused silica glass crucible, which otherwise can be used only for a single run of the CZ process due to cracking caused by a temperature decrease, can be re-used repeatedly for several runs. This process greatly decreases the manufacturing costs by improved productivity and cost saving of the expensive fused silica glass crucibles.
An alternative method of recharging is disclosed in Japanese Patent Kokai 62-260791 according to which the temperature of the melt remaining in the crucible after completion of a run of the CZ process for growing of a single crystal rod is slightly decreased so as to form a solidified crust on the melt surface. Silicon granules are introduced onto the crust through a feed conduit opening above the crucible.
In the above described recharging methods in general, it is a matter of course that the time taken for recharging of polycrystalline silicon desirably should be as short as possible in order to accomplish maximum improvement in productivity of the CZ process. Thus, the feed rate of the polycrystalline silicon for recharge must be as high as possible, provided that no damage is caused thereby in the fused silica glass crucible. In this regard, the polycrystalline silicon used for recharging is preferably in the form of a rod or block so as to enable the polycrystalline silicon to be introduced at one time or within a short period of time.
In the multiple CZ growth method taught by F. Shimura mentioned above, the polycrystalline silicon for recharge is in the form of a rod or block which is hung on the lower end of a pull-up shaft or wire and is introduced into the crucible by lowering the shaft or wire. It is natural in this case that the polycrystalline silicon charge can be performed only after complete removal of the already grown-up single crystal from above the crucible and can never be performed concurrently with the removal of the single crystal rod. Thus, the contribution of such method to improvement in productivity is limited.
In contrast thereto, the method taught in Japanese Patent Kokai 62-260891 can be performed concurrently with removal of the grown-up silicon single crystal rod so as to accomplish considerable improvement in productivity. This method, however, has some disadvantages. For example, the recharging of silicon granules can be performed only after crust formation on the surface of the silicon melt remaining in the crucible. Thus, a length of time is unavoidably required for cooling of the melt in the crucible. Further, formation of a crust on the surface of the melt causes mechanical damage at the inner surface of the crucible coming into contact with the crust. Thus, the desired advantage of cost saving for the expensive fused silica glass crucibles cannot be fully obtained.
The above mentioned problems relative to the recharging of silicon granules could be solved by improvement in feeding thereof into the crucible. When the feeder system for recharge is not provided with a mechanism for control of the feed rate of silicon granules, it is frequently the case that splashing of the silicon melt or bouncing of the granules onto the surface of the melt is caused by falling of the silicon granules. This problem of course can be at least partly solved by the use of a tapered feed conduit having a narrowed opening above the melt surface. The use of such a tapered feed conduit naturally has another problem in that the smooth falling of the silicon granules is more or less disturbed, so that the feed rate of the granules cannot always be high enough and can be controlled only with difficulty.